The Collaborative International Dictionary of English v.0.48:

Pignus \Pig"nus\, n.; pl. Pignora. [L.] (Rom. Law)
   A pledge or pawn.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

PIGNUS, civil law. This word signifies in English, pledge or pawn. (q.v.) It
is derived, says Gaius, from pugnium, the fist, because what is delivered in
pledge is delivered. in hand. Dig. 50, 16, 238, 2. This is one of several
instances of the failure of the Roman jurists, when they attempted
etymological explanation of words. The elements of pignus (pig) is contained
in the word p---[?], and its cognate forms. Smith's Dict. Gr. and Rom.
Antiq. h.v.